1. Field of the Invention
The present invention relates to an image forming apparatus adapted to fix a toner image to a recording medium by heating a toner borne on a recording medium as the toner image, as well as to a temperature control method for a fixing device of the image forming apparatus.
2. Description of the Related Art
The image forming apparatuses such as printers, copiers and facsimiles provide a permanent image on a recording medium, such as a paper sheet or a transparent sheet. The permanent image is provided by making a powder toner to adhere to the recording medium, in correspondence to an image to be formed, and then fusing the toner to the recording medium by applying heat thereto. As a fixing device of this type, there have heretofore been proposed various forms of devices, the commonest one of which is called a device of a fixing roller system (for example, a fixing device included in an image forming apparatus disclosed in Japanese Patent Publication No.3121495). In this system, the recording medium is passed through a nip between a heating roller heated to a predetermined temperature and a pressurizing roller for application of heat and pressure to the toner, thereby fixing the toner to the recording medium.
Further, the image forming apparatuses of this type include those which forms a color toner image by superimposing toner images of multiple different colors (e.g., yellow, magenta, cyan and black) on top of each other on an intermediate transfer medium rotatingly moved in a predetermined direction and then, transfers and fixes the resultant color toner image to the recording medium (for example, an image forming apparatus disclosed in Japanese Patent Application Laid-Open Gazette No.2001-290331). In this image forming apparatus, the toner images of yellow, magenta, cyan and black are sequentially formed on a photosensitive member. The individual toner images are primarily transferred to an intermediate transfer belt in a manner to be superimposed on top of each other on the intermediate transfer belt whereby the color toner image is formed on the intermediate transfer belt. The color toner image thus formed is secondarily transferred to the recording medium delivered from a cassette and then is delivered to a fixing device which, in turn, thermally fixes the color toner image on the recording medium.
The fixing device provided in such an image forming apparatus need to stabilize the temperature of the heating roller in a predetermined range in order to achieve a good fixing performance. Hence, a temperature control of the heating roller is performed by regulating the amount of electric power supplied to a heating element for heating the heating roller. More specifically, the temperature of the heating roller is sensed and a time period (duty) of energizing the heating element is determined based on the sensing result, the energization time period included in one cycle of a given control period (e.g., 1 second).
More recently, there is an increasing demand for faster image forming lower power consumption, which involves even further reduction of warm-up time (time required for temperature to reach a level to permit fixing) and power consumption of the fixing device. There is a tendency to use a heating roller having a relatively smaller heat capacity in order to meet such a demand. However, the reduction of the heat capacity of the heating roller entails a problem that the heating roller suffers relatively greater fluctuations of its temperature. Particularly when a recording medium at a low temperature (about room temperatures) is fed into the fixing device, the heating roller of the small heat capacity encounters an abrupt temperature drop. Consequently, image defects such as fixing failure and density variations may be produced, or the recording medium may be degraded due to overheating. That is, the fixing device may fail to achieve the good fixing performance.
On the other hand, the aforesaid image forming apparatus capable of forming a color image is provided with a sensor for detecting a reference position of the intermediate transfer belt. The apparatus utilizes a synchronous signal outputted from the sensor for superimposing the toner images of the multiple colors on top of each other as registering the images with one another. Specifically, the apparatus forms each toner image on the photosensitive member at each output of the synchronous signal from the sensor in a predetermined timing and then, primarily transfers the resultant toner image onto the intermediate transfer belt rotated at a given transport speed in synchronism with the photosensitive member. The apparatus controls the registration of the toner images during the primary transfer thereby achieving an accurate superimposition of the toner images of the multiple colors. Furthermore, the recording medium is delivered to a secondary transfer position based on the above synchronous signal so that a color toner image is secondarily transferred thereto. Therefore, so long as the intermediate transfer belt is rotatingly moved at a constant speed, the synchronous signal is outputted in a constant period. Hence, the formation of the toner image, the delivery of the recording medium and the secondary transfer of the toner image to the recording medium are carried out regularly.
In a practical image forming apparatus, however, a secondary transfer roller for performing the secondary transfer in a proper timing, a cleaner blade for cleaning a belt surface or the like temporarily abuts against the intermediate transfer belt during the formation of the color image. The abutment may interfere with the rotary transportation by the intermediate transfer belt, or cause elastic elongation of the intermediate transfer belt or similar elastic deformation of a drive system (such as a gear and a belt) for transmitting power to the intermediate transfer belt. Furthermore, the abutment applies a load to a belt drive section for driving the intermediate transfer belt into rotation. Hence, the intermediate transfer belt becomes incapable of being rotatingly moved at a constant speed due to the abutment or disengagement of such a component. As a result, a pre-fixing operation to be carried out prior to a fixing operation is carried out at an irregular time, the pre-fixing operation including the formation of the toner image, the delivery of the recording medium, the secondary transfer of the toner image to the recording medium and the like.
In a case where a conventional temperature control method providing a fixing temperature control in a constant period is directly applied to the aforementioned image forming apparatus, mismatch between the pre-fixing operation and the fixing operation may again cause the image defects such as fixing failure and density variations or the degradation of the recording medium due to overheating.